1. Field of the Invention
The present invention relates to an optical pickup apparatus that detects a tracking error signal and is not sensitive to the occurrence of an offset due to a shaking of an objective lens and a focus error signal in which the occurrence of an offset due to a deviation of a photodetector from a design location, a change in temperature, and/or a change in the wavelength of a light beam is reduced, and to a method of optimally focusing a light spot.
2. Description of the Related Art
Accurate detection of a focus and/or tracking error signal in recording and/or reproduction of a high capacity disc is necessary to perform a stable servo function. In general, an optical pickup apparatus includes a light source, an objective lens to focus a light beam emitted from the light source on a recording surface of an optical disc, and a light receiving optical system to detect an information signal and an error signal from a light beam reflected by the optical disc and having passed through the objective lens.
For an optical pickup apparatus in which a focus error signal is detected in an astigmatism method, the light receiving optical system is configured as shown in FIG. 1. Referring to FIG. 1, the light beam that is reflected by an optical disc (not shown) and having passed through an objective lens (not shown) is focused by a detection lens 2, passes through an adjustment lens 4 to adjust astigmatism of the reflected light beam, and is detected by a photodetector 6. As shown, the photodetector 6 has four light receiving areas A, B, C, and D arranged in a 2×2 matrix.
The light receiving areas A, B, C, and D and associated detection signals of the respective light receiving areas A, B, C, D are indicated by the same reference letters. An information signal RFS is detected by summing the detection signals of the light receiving areas A, B, C, and D, as shown in Equation 1:RFS=(A+B+C+D)  Equation 1
A focus error signal (FES) by the astigmatism method is detected by summing ones of the detection signals of the light receiving areas A, B, C, D adjacent in a diagonal direction and subtracting the summed signals, as shown in Equation 2:FES=(A+C)−(B+D)  Equation 2
A tracking error signal (TESpp) by a push-pull method is detected by summing ones of the detection signals of the light receiving areas A, B, C, D and adjacent and parallel to a track direction and subtracting the summed signals, as shown in Equation 3. Also, a tracking error signal (TESDPD) by a differential phase detection is detected by obtaining the phase of a sum signal of ones of the detection signals of the light receiving areas A, B, C, D adjacent in a diagonal direction and subtracting the obtained phases, as shown in Equation 3:TESpp=(A+D)−(B+C)  Equation 3TESDPD=phase(A+C)−phase(B+D)
Here, the push-pull method is used, for example, in the recording/reproduction of a DVD-RAM optical disc and in the recording of a DVD-R/RW optical disc. The differential phase detection method is used, for example, in the reproduction of a DVD-ROM optical disc and a DVD-R/RW optical disc.
An optical pickup apparatus having the conventional light receiving optical system has the following disadvantages. First, since the size of a light spot formed on the photodetector 6 is small, the focus error signal and tracking error signal are sensitive to a deviation of the photodetector 6 from a design location. Accordingly, an offset occurs in which the focus and tracking error signal is a value other than 0 at an on-focus and on-track position. Second, when the objective lens is shaken at the original position due to a seek or the decentering of an optical disc, the light beam is shifted on the photodetector 6 so that an offset is generated as to the push-pull signal. Third, when the focus error signal is detected with respect to a land/groove type optical disc, such as a DVD-RAM optical disc, by using the astigmatism method, cross talk is largely generated in the focus error signal due to the groove even in an on-focus state so that the optimal focus position of the land/groove is different. Fourth, the wavelength of a light beam emitted from the light source changes due to a change of temperature. Accordingly, the refractive power of the optical system changes. In most optical devices, the refractivity decreases as the wavelength increases. Thus, an offset is generated as to the focus error signal due to the change in temperature/wavelength.